Just checked my R25/2 which has that setup. You're right...you need a big external c-clip to hold the bolt affair to the lid. That's a pretty bit washer on the outside...guess it's needed. Hopefully, the threads will reach to the female threaded tang welded to the tank.

Well, you need some kind of protector between the bolt and the tank/paint. But if the washer is too thick, then you can't get the c-clip around the bolt shank. Beyond that, the exposed threads have to be able to reach the threaded tang in the tang.